Avalanche Forecast

Snow and Avalanche Discussion

After a week of widespread above freezing temperatures and a wet snow avalanche cycle that likely peaked last Sunday-Tuesday, we have turned the corner toward more typical cooler/showery spring-time weather as far as the higher terrain is concerned. In the adjacent Central-East zone, besides a loose wet cycle,  we saw wet snow avalanches release to the ground in shallow snowpack areas like Tumwater Canyon and the Wenatchee Foothills.

In the East South zone, many steep sunny aspects that previously had a shallow snowpack have significantly melted back. In the areas that still have a weak and shallow snowpack, stick to well-supported slopes and avoid areas of wet, unsupportive snow. You can check how deep water is traveling through the snowpack by digging and looking for moist layers of snow.

Spiral Butte, east of White Pass: Recent warm weather has significantly reduced snowpack on low elevations and on steep sun-affected slopes. Photo credit: Dallas Glass.

Loose wet avalanches: Cloud cover limits loose wet avalanche danger and we have a tricky cloud forecast on Sunday. Monitor changes in the upper snowpack, looking for natural pinwheels or small loose wet avalanches as clear signs of increasing danger. As the loose wet hazard increases, change your aspect and avoid similar slopes.

Other Spring Hazards

• Anywhere water is percolating through the snowpack, there is a potential for an unpredictable and dangerous wet slab avalanche. Keep this on your mind, especially when the sun comes out.

• There may be the potential for glide avalanches to release on steeper slopes with smooth underlying rock slabs.

• Despite the cooler weather, cornices are likely sagging and are most likely to fail when stressed by direct sunshine. Avoid traveling on cornices as they often break back further than you anticipate. Limit your exposure if traveling on slopes below large overhanging cornices.

• With water likely moving through the snowpack, particularly at lower elevations, creeks are now becoming exposed and difficult to cross. Plan your routes accordingly.

Forecast Schedule and No Rating

At this time, we do not have enough specific snowpack information to issue an avalanche hazard rating for the East Slopes South zone. However, even when No Rating is applied, applicable avalanche conditions and backcountry travel advice will be provided throughout the season. When weather systems produce very dangerous avalanche conditions in adjacent zones, NWAC will issue an avalanche warning for this zone as well.

March 22nd, 2019

Enter Spring

If you’ve been in the snow recently, the wintery conditions of early March may seem worlds away. You may be in for a surprise if it’s been a while since you were in the mountains. The weather has taken a turn towards spring in the last couple weeks and the Cascade snowpack the has undergone major changes. Unseasonably warm temperatures and strong sun followed a month-and-a-half of cold, winter storms. Mid-elevation weather stations stayed above freezing from March 15th-22nd with high temperatures reaching the upper 50’s to low 60’s. For an in-depth survey of the regional snowpack, we’ll divide the terrain up by aspect and elevation.

A graph showing temperatures between 4,000-5,000ft around the Cascades from the 16th-21st.

Aspects

Northerlies

Along with the warm temperatures, the spring sun has played a major role in warming snow surfaces. The result is a snowpack that varies by aspect. In most regions, shaded and northerly slopes remain relatively unchanged. Aside from some settlement and firmer or moist surfaces, the snow on north aspects is almost entirely dry. Even some low elevation north slopes are still holding snow.

Sunny slopes

The snowpack on east through south through west aspects is a different story. The strong March sun melted snow surfaces and drove melt-water into the snowpack. This is most dramatic on steep (over 35 degrees) southeast through southwest slopes below 5,000ft.  In some areas, you can find meltwater up to 3 feet below the snow surface with drainage channels well established. Between this warm period and rain events in the first half of the winter, the entire snowpack has transformed to melt forms. An important point to note is that as of the 22nd, these solar aspects remain unfrozen and weak. Cooler weather ahead may help strengthen moist to wet layers.

A glide avalanche (D2) released from a rock slab late on the 20th. Lichtenberg Mtn, 5,100ft, SE aspect. Other glide avalanches occurred on the 20th at Snoqualmie Pass and in Tumwater Canyon. Photo: Josh Hirshberg

Elevation

Low elevations

As you travel from low valleys to higher peaks, you’ll notice a major difference in the snowpack based on elevation. With all the low-elevation snow this winter, there are still some cold, shaded slopes holding pockets of snow down to 1,000ft, especially east of the Cascade Crest. However, most slopes below 3,000ft have lost much of their snow cover. Many low elevation, sun-exposed slopes are bare, especially in areas that previously held less than 3 feet of snow. The low elevation snowpack is no longer substantial enough to allow for easy travel over snow or widespread avalanches.

Loose wet avalanches on the south side of Table Mtn, near Mt Baker. 3/17. Photo: Pete Durr

Mid-elevations

At mid-elevations, around 3,000-5,000ft, the snowpack is still deep and layered. Many slopes at this elevation band near and west of the Cascade Crest are holding 6-10 feet of snow. This is also where you’ll find the most dramatic variation in the snowpack based on aspect.

High-elevations

Above 5,000ft you’ll encounter a snowpack similar to what you may have found around the 1st of March. Upper elevations have stayed mostly dry. The most sun-exposed slopes have surface crusts but have not seen much water or change to melt forms below the surface.